Semiconductor Heterostructure with Stress Management

ABSTRACT

A heterostructure for use in fabricating an optoelectronic device is provided. The heterostructure includes a layer, such as an n-type contact or cladding layer, that includes thin sub-layers inserted therein. The thin sub-layers can be spaced throughout the layer and separated by intervening sub-layers fabricated of the material for the layer. The thin sub-layers can have a distinct composition from the intervening sub-layers, which alters stresses present during growth of the heterostructure.

REFERENCE TO RELATED APPLICATIONS

The current application is a continuation of U.S. patent applicationSer. No. 14/686,845, which was filed on 15 Apr. 2015, and which claimsthe benefit of U.S. Provisional Application No. 61/979,717, which wasfiled on 15 Apr. 2014, each of which is hereby incorporated byreference.

TECHNICAL FIELD

The disclosure relates generally to semiconductor structures, and moreparticularly, to reducing internal stresses and/or wafer bowing duringepitaxial growth of semiconductor layers in a semiconductor structure.

BACKGROUND ART

Group III nitride semiconductors are widely used for fabricatingefficient blue and ultraviolet light emitting devices (e.g., diodes,lasers, etc.), ultraviolet detectors, and field effect transistors. Dueto a wide band-gap, these materials are a leading choice for fabricatingdeep ultraviolet light emitting diodes (DUV LEDs). In recent years,significant advances have been made in improving the efficiency of DUVLEDs. However, overall efficiencies of these devices remain low. Forfabrication of DUV LEDs, achieving a high quality aluminum nitride (AlN)buffer layer as an underlying layer can be important for the subsequentgrowth of any Al-rich group III nitride semiconductor layers. However,growth of an AlN layer with high crystal quality on substrates formed ofsapphire, silicon carbide (SiC) and silicon, which are currently themain substrates for growth of group III nitride devices, is extremelydifficult.

For light emitting devices, such as light emitting diodes (LEDs) andespecially deep ultraviolet LEDs (DUV LEDs), minimizing a dislocationdensity and a number of cracks in the semiconductor layers increases theefficiency of the device. In addition, it can lead to increasedreliability of the device. To this extent, several approaches havesought to grow low-defect semiconductor layers on patterned substrates.These approaches typically rely on reducing stresses present inepitaxially grown semiconductor layers.

For example, one approach to reduce stress accumulation in anepitaxially grown layer relies on patterning the underlying substrateusing microchannel epitaxy (MCE). Using MCE, a narrow channel is used asa nucleation center containing low defect information from thesubstrate. An opening in a mask acts as a microchannel, which transferscrystal information to the overgrown layer, while the mask preventsdislocations from transferring to the overgrown layer. As a result, theovergrown layer can become dislocation free. The three-dimensionalstructure of the MCE also provides another advantage to stress release.The residual stress can be released effectively since the overgrownlayer easily deforms. In another approach, a mask is applied at alocation of a large concentration of dislocation densities to blocktheir further propagation.

Other approaches rely on epitaxially growing a group III nitride basedsemiconductor superlattice. A superlattice structure mitigates thestrain difference between an aluminum nitride (AlN)/sapphire templateand the subsequent thick Al_(x)Ga_(1−x)N (where 0≦x≦1) layers. Fordevices such as DUV LEDs, thick AlGaN epitaxial layers (e.g., on theorder of a few micrometers) are desirable to reduce current crowding.Using a superlattice approach, an AlN/AlGaN superlattice was grown toreduce biaxial tensile strain and a 3.0 μm-thick Al_(0.2)Ga_(0.8)N wasachieved on sapphire without any cracks. Such a superlattice can be usedto minimize the dislocation density due to varying stresses in thesub-layers of the superlattice elements.

While the superlattice approaches allow some control of tensile andcompressive stresses in epitaxially grown nitride semiconductor layers,the approaches do not enable epitaxial growth of nitride basedsemiconductor layers with uniform composition. Based on previousexperience obtained from gallium nitride (GaN) growth, lateral epitaxialovergrowth (LEO) has proven an efficient way for significant reductionof dislocation in GaN films. Several other technologies evolved fromLEO, such as pendeo-epitaxial, cantilever epitaxy, and facet controlledLEO, have also been developed. While the above approaches work well forepitaxial growth of GaN semiconductor layers, epitaxial growth ofaluminum nitride (AlN) layers is more challenging due to a relativelysmall lateral growth of AlN films.

Another leading approach includes growth of AlN films over patternedsubstrates, such as, for example, a patterned sapphire substrate (PSS).While the PSS-based approach generally produces an AlN layer withreduced stress and low dislocation densities, the patterning process andsubsequent growth of AlN films is technologically complicated andcostly.

SUMMARY OF THE INVENTION

Aspects of the invention provide a heterostructure for use infabricating an optoelectronic device. The heterostructure includes alayer, such as an n-type contact or cladding layer, that includes thinsub-layers inserted therein. The thin sub-layers can be spacedthroughout the layer and separated by intervening sub-layers fabricatedof the material for the layer. The thin sub-layers can have a distinctcomposition from the intervening sub-layers, which alters stressespresent during growth of the heterostructure. The layer can beconfigured to control stresses present during growth of theheterostructure. To this extent, the thin sub-layers can be configuredto reduce internal stresses, wafer bowing, and/or the like.

A first aspect of the invention provides a heterostructure comprising: asubstrate; and a group III nitride layer epitaxially grown on thesubstrate, wherein the group III nitride layer includes: a plurality ofsub-layers of a first group III nitride material and having a firstthickness; and a plurality of thin sub-layers of a second group IIInitride material and having a second thickness, wherein the plurality ofsub-layers alternate with the plurality of thin sub-layers, wherein thefirst group III nitride material includes a molar fraction of gallium ofat least 0.05, wherein a molar fraction of gallium in the first groupIII nitride material differs from the molar fraction of gallium in thesecond group III nitride material by at least 0.05, and wherein thesecond thickness is at most five percent of the first thickness.

A second aspect of the invention provides an optoelectronic devicecomprising: a substrate; and an n-type layer formed of group III nitridematerials, wherein the n-type layer includes: a plurality of sub-layersof a first group III nitride material and having a first thickness; anda plurality of thin sub-layers of a second group III nitride materialand having a second thickness, wherein the plurality of sub-layersalternate with the plurality of thin sub-layers, wherein the first groupIII nitride material includes a molar fraction of gallium of at least0.05, wherein a molar fraction of gallium in the first group III nitridematerial differs from the molar fraction of gallium in the second groupIII nitride material by at least 0.05, and wherein the second thicknessis at most five percent of the first thickness.

A third aspect of the invention provides a method of fabricating adevice, the method comprising: epitaxially growing a group III nitridelayer on a substrate, wherein the group III nitride layer includes: aplurality of sub-layers of a first group III nitride material and havinga first thickness; and a plurality of thin sub-layers of a second groupIII nitride material and having a second thickness, wherein theplurality of sub-layers alternate with the plurality of thin sub-layers,wherein the first group III nitride material includes a molar fractionof gallium of at least 0.05, wherein a molar fraction of gallium in thefirst group III nitride material differs from the molar fraction ofgallium in the second group III nitride material by at least 0.05, andwherein the second thickness is at most five percent of the firstthickness.

The illustrative aspects of the invention are designed to solve one ormore of the problems herein described and/or one or more other problemsnot discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIG. 1 shows a schematic structure of an illustrative optoelectronicdevice according to an embodiment.

FIGS. 2A and 2B show illustrative heterostructures for use infabricating an optoelectronic device according to the prior art and anembodiment, respectively.

FIG. 3 shows average sheet resistance of an AlGaN layer as a function ofthin AlN sub-layer thickness according to embodiments.

FIG. 4 shows an amount of bowing of a substrate wafer as a function ofthin AlN sub-layer thickness according to embodiments.

FIG. 5 shows an effect of AlN sub-layer thickness on the latticeconstant a of several semiconductor layers in a heterostructureaccording to an embodiment.

FIGS. 6A and 6B show illustrative stress diagrams for bowed wafers atroom temperature according to the prior art and an embodiment,respectively.

FIGS. 7A and 7B show surface morphologies resulting from growth withoutand with thin sub-layers described herein according to the prior art andan embodiment, respectively.

FIG. 8 shows illustrative plots of the lattice constants a and c as afunction of the V/III ratio for an AlN layer according to an embodiment.

FIG. 9 shows illustrative plots of stress and strain as a function ofthe V/III ratio for an AlN layer epitaxially grown on a sapphiresubstrate according to an embodiment.

FIGS. 10A and 10B show illustrative n-type layers according toembodiments.

FIG. 11 shows an illustrative flow diagram for fabricating a circuitaccording to an embodiment.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a heterostructurefor use in fabricating an optoelectronic device. The heterostructureincludes a layer, such as an n-type contact or cladding layer, thatincludes thin sub-layers inserted therein. The thin sub-layers can bespaced throughout the layer and separated by intervening sub-layersfabricated of the material for the layer. The thin sub-layers can have adistinct composition from the intervening sub-layers, which altersstresses present during growth of the heterostructure. The layer can beconfigured to control stresses present during growth of theheterostructure. To this extent, the thin sub-layers can be configuredto reduce internal stresses and/or wafer bowing. As used herein, unlessotherwise noted, the term “set” means one or more (i.e., at least one)and the phrase “any solution” means any now known or later developedsolution.

The present technique may be combined with the technique of growth of abuffer layer as described in U.S. Provisional Patent Application No.61/943,365, filed on 22 Feb. 2014, and U.S. Utility patent applicationSer. No. 14/628,281, filed on 22 Feb. 2015, both of which are herebyincorporated by reference.

Turning to the drawings, FIG. 1 shows a schematic structure of anillustrative optoelectronic device 10 according to an embodiment. In amore particular embodiment, the optoelectronic device 10 is configuredto operate as an emitting device, such as a light emitting diode (LED)or a laser diode (LD). In either case, during operation of theoptoelectronic device 10, application of a bias comparable to the bandgap results in the emission of electromagnetic radiation from an activeregion 18 of the optoelectronic device 10. The electromagnetic radiationemitted (or sensed) by the optoelectronic device 10 can have a peakwavelength within any range of wavelengths, including visible light,ultraviolet radiation, deep ultraviolet radiation, infrared light,and/or the like. In an embodiment, the device 10 is configured to emit(or sense) radiation having a dominant wavelength within the ultravioletrange of wavelengths. In a more specific embodiment, the dominantwavelength is within a range of wavelengths between approximately 210and approximately 360 nanometers. In a still more specific embodiment,the dominant wavelength is approximately 280 nanometers.

The optoelectronic device 10 includes a heterostructure 11 comprising asubstrate 12, a buffer layer 14 adjacent to the substrate 12, an n-typelayer 16 (e.g., a cladding layer, electron supply layer, contact layer,and/or the like) adjacent to the buffer layer 14, and an active region18 having an n-type side adjacent to the n-type layer 16. Furthermore,the heterostructure 11 of the optoelectronic device 10 includes a firstp-type layer 20 (e.g., an electron blocking layer, a cladding layer,hole supply layer, and/or the like) adjacent to a p-type side of theactive region 18 and a second p-type layer 22 (e.g., a cladding layer,hole supply layer, contact layer, and/or the like) adjacent to the firstp-type layer 20.

In a more particular illustrative embodiment, the optoelectronic device10 is a group III-V materials based device, in which some or all of thevarious layers are formed of elements selected from the group III-Vmaterials system. In a still more particular illustrative embodiment,the various layers of the optoelectronic device 10 are formed of groupIII nitride based materials. Group III nitride materials comprise one ormore group III elements (e.g., boron (B), aluminum (Al), gallium (Ga),and indium (In)) and nitrogen (N), such that B_(w)Al_(x)Ga_(y)In_(z)N,where 0≦W, X, Y, Z≦1, and W+X+Y+Z=1. Illustrative group III nitridematerials include binary, ternary and quaternary alloys such as, AlN,GaN, InN, BN, AlGaN, AlInN, AlBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBNwith any molar fraction of group III elements.

An illustrative embodiment of a group III nitride based optoelectronicdevice 10 includes an active region 18 (e.g., a series of alternatingquantum wells and barriers) composed of In_(y)Al_(x)Ga_(1−x−y)N,Ga_(z)In_(y)Al_(x)B_(1−x−y−z)N, an Al_(x)Ga_(1−x)N semiconductor alloy,or the like. Similarly, the n-type layer 16, the first p-type layer 20,and the second p-type layer 22 can be composed of anIn_(y)Al_(x)Ga_(1−x−y)N alloy, a Ga_(z)In_(y)Al_(x)B_(1−x−y−z)N alloy,or the like. The molar fractions given by x, y, and z can vary betweenthe various layers 16, 18, 20, and 22. When the optoelectronic device 10is configured to be operated in a flip chip configuration, such as shownin FIG. 1, the substrate 12 and buffer layer 14 should be transparent tothe target electromagnetic radiation. To this extent, an embodiment ofthe substrate 12 is formed of sapphire, and the buffer layer 14 can becomposed of AlN, an AlGaN/AlN superlattice, and/or the like. However, itis understood that the substrate 12 can be formed of any suitablematerial including, for example, silicon carbide (SiC), silicon (Si),bulk GaN, bulk AlN, bulk or a film of AlGaN, bulk or a film of BN, AlON,LiGaO₂, LiAlO₂, aluminum oxinitride (AlO_(x)N_(y)), MgAl₂O₄, GaAs, Ge,or another suitable material. Furthermore, a surface of the substrate 12can be substantially flat or patterned using any solution.

The optoelectronic device 10 can further include a p-type contact 24,which can form an ohmic contact to the second p-type layer 22, and ap-type electrode 26 can be attached to the p-type contact 24. Similarly,the optoelectronic device 10 can include an n-type contact 28, which canform an ohmic contact to the n-type layer 16, and an n-type electrode 30can be attached to the n-type contact 28. The p-type contact 24 and then-type contact 28 can form ohmic contacts to the corresponding layers22, 16, respectively.

In an embodiment, the p-type contact 24 and the n-type contact 28 eachcomprise several conductive and reflective metal layers, while then-type electrode 30 and the p-type electrode 26 each comprise highlyconductive metal. In an embodiment, the second p-type layer 22 and/orthe p-type electrode 26 can be transparent to the electromagneticradiation generated by the active region 18. For example, the secondp-type layer 22 and/or the p-type electrode 26 can comprise a shortperiod superlattice lattice structure, such as an at least partiallytransparent magnesium (Mg)-doped AlGaN/AlGaN short period superlatticestructure (SPSL). Furthermore, the p-type electrode 26 and/or the n-typeelectrode 30 can be reflective of the electromagnetic radiationgenerated by the active region 18. In another embodiment, the n-typelayer 16 and/or the n-type electrode 30 can be formed of a short periodsuperlattice, such as an AlGaN SPSL, which is transparent to theelectromagnetic radiation generated by the active region 18.

As further shown with respect to the optoelectronic device 10, thedevice 10 can be mounted to a submount 36 via the electrodes 26, 30 in aflip chip configuration. In this case, the substrate 12 is located onthe top of the optoelectronic device 10. To this extent, the p-typeelectrode 26 and the n-type electrode 30 can both be attached to asubmount 36 via contact pads 32, 34, respectively. The submount 36 canbe formed of aluminum nitride (AlN), silicon carbide (SiC), and/or thelike, which can provide improved heat management during operation of theoptoelectronic device 10.

It is understood that the particular configuration and arrangement oflayers shown for the optoelectronic device 10 are only illustrative ofvarious configurations and arrangements that are possible underembodiments of the invention. For example, a device heterostructure caninclude one or more additional layers located between the substrate 12and the n-type contact layer 16. Similarly, a device heterostructure caninclude one or more additional layers, each of which is designed toprovide a particular function relating to the overall operation of thedevice (e.g., wave guiding, radiation extraction, electron/holeblocking, breakdown prevention, and/or the like).

In general, growth of the n-type contact layer 16 followed by growth ofadditional layers, such as the active region 18 and the p-type layer(s)20, 22, leads to concave bowing of the heterostructure during growth.The concave bowing changes to convex bowing during cool down (aftergrowth) due to a difference in thermal expansion coefficient between theepitaxially grown layers and the substrate 12. For example, the thermalexpansion coefficient of the substrate 12 may be larger than that of theepitaxially grown semiconductor layers. In a more particular example,for growing group III nitride semiconductor layers on a sapphiresubstrate 12, the thermal expansion coefficient of sapphire is about8×10⁻⁶, whereas the thermal expansion coefficient of the epitaxiallygrown layers can be half as much as the thermal expansion coefficient ofthe sapphire substrate 12.

The inventors propose to modify prior art heterostructures to provide animproved stress configuration in the heterostructure at roomtemperature. To achieve a desirable stress configuration at roomtemperature, concave bowing is preferred during the growth, where theconcave bowing is a result of tensile stresses present during thegrowth. During the cool down, the substrate 12 undergoes a large thermalcontraction resulting in additional compressive stresses within thesemiconductor layers. The inventors propose to obtain a more optimalstress configuration by providing more balanced tensile and compressivestresses during growth and cool down. Unfortunately, it is not readilyfeasible to produce a heterostructure exhibiting no stresses. Inparticular, compressive stresses in the film generated during cool downare typically larger than tensile stresses generated during growth. As aresult, at room temperature, the substrate 12 is typically convex.

For generation of reliable devices 10 and for subsequent processing ofthe wafer, an amount of convex bowing of the wafer at room temperaturecan be kept as small as possible. This goal often results in growingepitaxial layers having large tensile stresses and concave bowing duringgrowth. Unfortunately, growing thick layers having tensile stresses canresult in either cracking or relaxation through the propagation ofdislocations within the layers. Once the tensile stresses are relaxed,they cannot counterbalance subsequent compressive stresses during thecool down.

In addition to controlling stresses in the semiconductor layers,reliability of a group III nitride-based light emitting device 10depends on the number of dislocations present in the layers, and morespecifically in the active region 18 of the device 10. Typically, forthick layers, the dislocation density is substantially reduced due tolayer relaxation through dislocation annihilation and bending.Unfortunately, thick semiconductor layers lead to increased stresses,which if not kept under control may relax through generation and/orpropagation of dislocations into the active region 18.

FIGS. 2A and 2B show illustrative heterostructures 1, 11, respectively,for use in fabricating an optoelectronic device according to the priorart and an embodiment, respectively. As shown in FIG. 2A, a prior artheterostructure 1 can include a substrate 2 on which a buffer layer 4(e.g., a nucleation layer) is epitaxially grown. An intermediate layer 5can be grown on the buffer layer 4, which can be followed by growth ofan n-type contact layer 6. For fabricating a group III nitride-baseddevice, the buffer layer 4 can be formed of aluminum nitride, theintermediate layer 5 can be a superlattice structure including pairs ofAl_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N layers, and the n-type contact layer 6can be formed of doped aluminum gallium nitride.

In general, the n-type contact layer 6 can be an Al_(x)Ga_(1−x)N layer,in which the amount of aluminum (the x value) can be tailored dependingon the design of the active region to be formed thereon. For example,for an active region designated to radiate at a peak wavelength of about280 nm, the n-type contact layer 6 can have x within a range of0.35-0.65, and typically has value of about 0.5. The n-type contactlayer 6 is typically one of the thickest layers in the heterostructure1. This layer 6 can have a large impact on the performance and thereliability of the corresponding device.

The inventors propose to control stress in an n-type contact layer byinserting thin sub-layers of a different composition in the n-typecontact layer. While aspects of the invention are described withreference to an n-type contact layer, it is understood that embodimentscan be directed to various types of layers including, for example, othern-type layers such as an n-type cladding layer, an electron supplylayer, a buffer layer 14, and/or the like. Furthermore, an embodiment ofthe invention provides an intermediate layer 15 including thinsub-layers configured as described herein.

Regardless, FIG. 2B shows a heterostructure 11 according to anembodiment. As illustrated, the heterostructure 11 includes a substrate12, a buffer layer 14, an intermediate layer 15 (shown as a superlatticestructure), and an n-type contact layer 16. The n-type contact layer 16includes a plurality of thin sub-layers, such as the thin sub-layers40A, 40B, which are inserted in the n-type contact layer 16 andseparated by intervening sub-layers, such as the intervening sub-layers42A, 42B, which are formed of the n-type contact layer 16 material. Inthis case, a total number of the thin sub-layers 40A, 40B, a frequencyof insertion of the thin sub-layers 40A, 40B (e.g., as characterized bythe relative thicknesses of the n-type contact layer 16 and theintervening sub-layers 42A, 42B), as well as a thickness of the insertedsub-layers 40A, 40B can be adjusted to control an amount of stress inthe n-type contact layer 16. An active region 18 (FIG. 1), can be grownon the n-type contact layer 16, and can comprise, for example, a seriesof quantum wells and barriers, which are configured to emit radiation ofa target peak wavelength during operation of the corresponding device.

In an embodiment, the heterostructure 11 is a group III nitride-basedheterostructure. In this case, the n-type contact layer 16, andtherefore the intervening sub-layers 42A, 42B, can be formed of a groupIII nitride material including gallium. For example, the group IIInitride material of the intervening sub-layers 42A, 42B can beAl_(x)In_(y)B_(z)Ga_(1−x−y−z)N, where 0≦x, y, z<1 and 1−x−y−z≧0.05. In amore particular embodiment, the group III nitride material of theintervening sub-layers 42A, 42B is AlGaN. In a still more particularembodiment, the group III nitride material of the intervening sub-layers42A, 42B is Al_(x)Ga_(1−x)N with 0.4<x<0.7. The intervening sub-layers42A, 42B also can be doped with an n-type dopant. In an embodiment, theintervening sub-layers 42A, 42B have an n-type dopant concentration ofat least 5×10¹⁷ dopant atoms per cubic centimeter.

The thin sub-layers 40A, 40B can be formed of a group III nitridematerial including less gallium than that included in the interveningsub-layers 42A, 42B. In an embodiment, the thin sub-layers 40A, 40B areformed of a group III nitride material in which the gallium molarfraction is at least 0.05 lower than the gallium molar fraction in theintervening sub-layers 42A, 42B. In a more particular embodiment, thegroup III nitride material of the thin sub-layers 40A, 40B isAl_(y)Ga_(1−y)N, where 0<y≦1. In a still more particular embodiment, thethin sub-layers 40A, 40B are formed of aluminum nitride. In a particularembodiment, a thin sub-layer 40A, 40B can comprise a monolayer.

The thin sub-layers 40A, 40B have a thickness significantly less thanthe thickness of the intervening sub-layers 42A, 42B. In an embodiment,each intervening sub-layer 42A, 42B has a thickness of at least twentynanometers. In a more particular embodiment, each intervening sub-layer42A, 42B has a thickness of between approximately 20 nanometers andapproximately 500 nanometers. In an embodiment, the thin sub-layers 40A,40B have a thickness that is at most five percent a thickness of theintervening sub-layers 42A, 42B. In a more particular embodiment, thethin sub-layers 40A, 40B have a thickness that is at most two percent athickness of the intervening sub-layers 42A, 42B. In an embodiment, athickness of each of the thin sub-layers 40A, 40B is in a range betweenapproximately 0.2 nanometers and approximately 2 nanometers. In a moreparticular embodiment, a thickness of each of the thin sub-layers 40A,40B is in a range between approximately 0.2 nanometers and approximately1 nanometer. In a still more particular embodiment, a thickness of eachof the thin sub-layers 40A, 40B is in a range between approximately 0.2nanometers and approximately 0.6 nanometers. In an embodiment, the thinsub-layers 40A, 40B can be configured to have a thickness on the orderof or less than a tunneling length of carriers in the n-type layer 16.

An illustrative configuration of the n-type layer 16 includesintervening sub-layers 42A, 42B, each formed of Al_(x)Ga_(1−x)N, with0.4<x<0.7 and having a thickness in the range of 20-500 nm and thinsub-layers 40A, 40B, each of which is formed of AlN and has a thicknessin a range of 0.2-2 nm. Considering a heterostructure 11 used tofabricate an optoelectronic device 10 (FIG. 1) configured to operate ata peak wavelength of approximately 280 nanometers as a more particularexample, the intervening sub-layers 42A, 42B can be formed ofAl_(0.5)Ga_(0.5)N and the thin sub-layers 40A, 40B can be formed of AlN.However, it is understood that the molar fraction of aluminum can be ina range of 0.35 to 0.65. Depending on the peak wavelength for theoptoelectronic device 10, the composition of the intervening sub-layers42A, 42B can vary. For example, the composition of the interveningsub-layers 42A, 42B, or at least the intervening sub-layer closest tothe active region 18 (FIG. 1), can be selected to be within twentypercent of the composition of the barriers of the active region 18 (orat least the barrier closest to the n-type contact layer 16), which willgenerally change depending on the peak wavelength of radiation emittedby the active region 18. To this extent, a spacing between the thinsub-layers 40A, 40B (e.g., a thickness of the intervening sub-layers42A, 42B) can be adjusted based on the operating wavelength of thedevice 10 (FIG. 1). For example, for an optoelectronic device 10including an n-type contact layer 16 having intervening sub-layers 42A,42B formed of Al_(0.3)Ga_(0.7)N, the intervening sub-layers 42A, 42B canhave thicknesses of approximately 25 nanometers. However, for anoptoelectronic device 10 including an n-type contact layer 16 havingintervening sub-layers 42A, 42B formed of Al_(x)Ga_(1−x)N, with0.5≦x≦0.6, the intervening sub-layers 42A, 42B can have thicknesses ofapproximately 50 nanometers.

More generally, thicknesses of the intervening sub-layers 42A, 42B canvary approximately linearly based on the molar fraction of aluminum. Forexample, for a molar fraction of aluminum of about 0.3, the thicknessesof the intervening sub-layers 42A, 42B can be about 30 nanometers,whereas for a molar fraction of aluminum of about 0.6, the thicknessesof the intervening sub-layers 42A, 42B can be about 60 nanometers.However, it is understood that the various thicknesses and molarfractions can vary by as much as fifty percent from the values stated.

While the n-type contact layer 16 is primarily described as being formedof AlGaN intervening sub-layers 42A, 42B, it is understood that then-type contact layer 16 can be another type of group III nitridematerial. For example, the n-type contact layer 16 can include, forexample, indium and/or boron, a different molar fraction of aluminum,and/or the like, which can be selected depending on the functionality ofthe device 10. Furthermore, such a device is only illustrative ofvarious types of optoelectronic devices 10, including optoelectronicdevices 10 configured to operate at different peak wavelengths. In analternative embodiment, a device can comprise an electronic device, suchas a high electron mobility transistor. In this case, a thick GaN-basedcarrier electronic layer can include the inserted thin sub-layers 40A,40B as described herein in conjunction with the n-type contact layer 16.

The inventors fabricated various heterostructures 11 with thinsub-layers 40A, 40B of different thicknesses, insertion frequencies, andperiods (e.g., total number of thin sub-layers 40A, 40B), and evaluatedthe conductivity of the resulting Al_(0.6)Ga_(0.4)N (e.g., n-typecontact or AlGaN) layers 16. In each case, the thin sub-layers 40A, 40Bwere formed of AlN. FIG. 3 shows average sheet resistance of an AlGaNlayer 16 as a function of thin AlN sub-layer 40A, 40B thicknessaccording to embodiments. As illustrated, sub-layers 40A, 40B havingthicknesses of approximately 0.5 nanometers or less do not significantlyincrease (e.g., the increase is less than 100 ohms/square) the averagesheet resistance of the AlGaN layer 16. However, AlN sub-layers 40A, 40Bhaving a thickness of approximately 1 nanometer cause an approximatelythree times increase in the sheet resistance of the AlGaN layer 16 overthat of an AlGaN layer 6 without any AlN sub-layers 40A, 40B. In anembodiment, the thicknesses of the thin sub-layers 40A, 40B are selectedto limit the sheet resistivity of the layer 16 to no more than fiftypercent more than the sheet resistivity of a layer 6 (FIG. 2A) havingthe same thickness as the layer 16 and formed of the same material asthe intervening sub-layers 42A, 42B, but without the thin sub-layers40A, 40B.

In an embodiment, the thicknesses of the sub-layers 40A, 40B, 42A, 42Band the total number of thin sub-layers 40A, 40B can be selected tominimize the stresses present in the heterostructure 11 after growth.The stresses can be assessed, for example, by measuring wafer bowing atroom temperature, after completion of the growth of the heterostructure11. In an embodiment, the thicknesses of the sub-layers 40A, 40B, 42A,42B and the total number of thin sub-layers 40A, 40B are selected toreduce an amount of wafer bowing by at least ten percent over an amountof wafer bowing that would occur in a comparable prior artheterostructure 1 (FIG. 2A).

Additionally, wafer bowing can be monitored during the growth to ensurethat abrupt changes in the amount of bowing do not occur, which isindicative of the heterostructure cracking. In an embodiment, thethicknesses of the sub-layers 40A, 40B, 42A, 42B and the total number ofthin sub-layers 40A, 40B are selected to maximize tensile stressespresent during growth without causing the formation of cracks or strongrelaxation in the heterostructure 11. An amount of tensile stress can beassessed, for example, by measuring wafer bowing during growth, andwhere the formation of cracks or strong relaxation is assessed byobserving abrupt changes in the wafer bowing. In a further embodiment,an abrupt change to wafer bowing can be defined as a change in wafercurvature of at least fifty 1/km per ten minutes of growth time. In anembodiment, the thicknesses of the sub-layers 40A, 40B, 42A, 42B areselected to result in concave bowing of at least 100 1/km during growthof the heterostructure 11.

FIG. 4 shows an amount of bowing of a substrate wafer including aheterostructure 11 as a function of thin AlN sub-layer thickness 40A,40B according to embodiments. In this case, bowing is defined as thevertical displacement of the center of the wafer from the edges of thewafer. The inventors fabricated several versions in which the frequencyof the AlN sub-layers 40A, 40B placed within the corresponding AlGaNlayer 16 were varied by varying the number of periods (loops). In eachcase, the AlGaN layer 16 has a thickness of approximately two microns.As illustrated, an amount of bowing is minimized when the AlN sub-layers40A, 40B have a thickness of approximately 0.5 nanometers. Furthermore,at least for a total number of sub-layers 40A, 40B exceeding twenty, ahigher number of sub-layers 40A, 40B generally resulted in an increasedbowing over that provided by the lower number of sub-layers 40A, 40B foran AlGaN layer 16 of the same thickness. In an embodiment, a total oftwenty to forty thin sub-layers 40A, 40B are inserted for a two micronthick layer 16. In another embodiment an n-type contact layer 16 havinga thickness between two to three microns includes approximately 80 thinsub-layers 40A, 40B, but no more than 120 thin sub-layers 40A, 40B.

FIG. 5 shows an effect of AlN sub-layer thickness on the latticeconstant a of several semiconductor layers in a heterostructure 11according to an embodiment. In each case, the heterostructure 11includes an AlN buffer layer 14 and an intermediate layer 15 locatedbetween the AlN buffer layer 14 and an Al_(0.6)Ga_(0.4)N layer 16. Theintermediate layer 15 has first (SL1) and second (SL2) superlatticescomprising pairs of Al_(x1)Ga_(1−x1)N/Al_(y1)Ga_(1−y1)N layers andAl_(x2)Ga_(1−x2)N/Al_(y2)Ga_(1−y2)N, respectively. The horizontal dottedline corresponds to the lattice constant a for bulk Al_(0.6)Ga_(0.4)N.As illustrated, an AlN sub-layer thickness of approximately 0.5nanometers corresponds to a minimum lattice constant a in the AlGaNlayer, which indicates the best pseudomorphic fit and the lack ofrelaxation of the AlGaN material. As discussed herein, a large amount ofrelaxation is undesirable due to dislocation propagation and possiblecracking. To this extent, the lattice constant a for the AlGaN layerbeing comparable to that of bulk Al_(0.6)Ga_(0.4)N is indicative ofrelaxation having occurred.

FIGS. 6A and 6B show illustrative stress diagrams for bowed wafers atroom temperature according to the prior art and an embodiment,respectively. The wafers include heterostructures 1, 11 for use infabricating an optoelectronic device according to the prior art and anembodiment, respectively. An average stress in a semiconductor layer canbe computed by calculating the integral of the stress along thethickness of the layer, and such an average stress can be related towafer bowing through Stoney's formula, for example. As illustrated,while the sign of the stresses 5, 50 change from one laminate layer toanother, an overall averaged stress within a film can be deduced fromthe bowing of the wafer. For example, an average stress within anepitaxial semiconductor film is compressive as shown by the convexbowing. A local value of stress within the epitaxial laminate structurechanges from one film layer to another discontinuously along theinterface.

FIG. 6A shows a possible distribution of stresses 5 in a heterostructure1 that does not include the thin sub-layers described herein. In thiscase, the relaxation process in the n-type contact layer 6 (e.g., AlGaN)can induce tensile stresses within the n-type contact layer 6. Thesetensile stresses can lead to the formation of micro/nano cracks and/oran increase in a number of dislocations, e.g., as indicated by region 7.These cracks and/or dislocations result in reduced device reliability.However, as shown in FIG. 6B, inclusion of the thin sub-layers 40A, 40Bcan avoid relaxation, which prevents tensile stresses from occurring inthe n-type contact layer 16. As further shown in FIG. 6B, the thinsub-layers 40A, 40B have tensile stresses, while the interveningsub-layers 42A, 42B have compressive stresses. However, the thinsub-layers 40A, 40B do not exhibit significant relaxation and/or crackformation since they are thin (e.g., less than 2 nanometers thick).

To this extent, inclusion of the thin sub-layers 40A, 40B can improvethe morphology of one or more layers in the heterostructure. Forexample, FIGS. 7A and 7B show surface morphologies resulting from growthwithout and with thin sub-layers described herein according to the priorart and an embodiment, respectively. As illustrated, hexagonal defects(one of which is circled in FIG. 7A) are much more widespread in thelayer grown without the thin sub-layers than in the layer grown with thethin sub-layers. In an embodiment, the thicknesses of the sub-layers40A, 40B, 42A, 42B and the total number of thin sub-layers 40A, 40B areselected to reduce a number of defects and dislocations present in theactive region 18 (FIG. 1). In a more particular embodiment, thereduction is at least an order of magnitude as compared to dislocationspresent at the interface between the n-type contact layer 16 and theintermediate layer 15 or buffer layer 14. In an illustrative embodiment,the heterostructure 11 is configured to use in fabricating a lightemitting diode radiating at a peak wavelength in a range of 300-350nanometers. In this case, the intervening sub-layers 42A, 42B can begroup III nitride layers with an aluminum molar fraction in the range of0.1 to 0.4 and thicknesses the range of 10 to 100 nanometers, while thethin sub-layers 40A, 40B can be, for example, AlN, and have thicknessesin the range of 0.2 to 0.8 nanometers.

Growth of an n-type contact layer 16 described herein can be performedusing any solution. Additionally, it is understood that one or moreattributes of the growth solution and/or n-type layer 16 can be alteredduring growth of the n-type layer 16. Such alteration(s) can beconfigured to result in an n-type layer 16 having properties that changein a vertical and/or lateral direction. In an embodiment, a growthtemperature is changed for growth of the intervening sub-layers 42A, 42Band growth of the thin sub-layers 40A, 40B, e.g., to adjust a rate ofgrowth and/or quality of the corresponding semiconductor material. Forexample, the respective growth temperatures can differ by at least 100degrees Celsius. In a more particular example, the growth temperaturefor the intervening sub-layers 42A, 42B is at least 100 degrees Celsiushigher than the growth temperature for the thin sub-layers 40A, 40B. Amolar ratio between group V precursors and group III precursors (theV/III ratio) also can differ for growth of the intervening sub-layers42A, 42B and growth of the thin sub-layers 40A, 40B, e.g., to affect thequality and/or unintentional doping of the semiconductor material. In anembodiment, the respective V/III ratios differ by at least five percent.

Additionally, one or more attributes of the n-type contact layer 16 canbe changed during growth of the n-type contact layer 16. For example, inan embodiment, the thicknesses of the various sub-layers 40A, 40B, 42A,42B can remain substantially constant throughout the n-type contactlayer 16 (e.g., the thin sub-layers 40A, 40B are inserted insubstantially constant intervals). Alternatively, the thicknesses of theintervening sub-layers 42A, 42B (and therefore the insertion frequencyof the thin sub-layers 40A, 40B) and/or compositions of the sub-layers40A, 40B, 42A, 42B can change throughout the n-type contact layer 16. Inan embodiment, the thickness of each intervening sub-layer 42A, 42Bincreases in a direction away from the substrate 12, thereby causing theinsertion frequency of the thin sub-layers 40A, 40B to be higher closerto the substrate 12 and lower closer to the active region 18. In a moreparticular embodiment, the thicknesses increase by at least five percentbetween adjacent intervening sub-layers 42A, 42B. Furthermore, then-type contact layer 16 can include multiple distinct regions, each witha different configuration (e.g., insertion frequency) of thin sub-layers40A, 40B. To this extent, the n-type contact layer 16 is shown includinga region 44, which includes no thin sub-layers 40A, 40B. In anembodiment, the region 44 is fabricated from the same material, but hasa larger thickness than the intervening sub-layers 42A, 42B.

In an embodiment, a composition of comparable sub-layers 40A, 40B and/or42A, 42B changes throughout the n-type contact layer 16. In a moreparticular embodiment, the compositions change by at least two percentbetween two adjacent comparable sub-layers 40A, 40B and/or 42A, 42B. Forexample, an aluminum composition in the intervening sub-layers 42A, 42Bcan decrease in each subsequent intervening sub-layer 42A, 42B grown ina direction away from the substrate 12.

In an embodiment, a thickness of a thin sub-layer 40A, 40B varies in alateral direction. For example, a lateral scale of the thicknessfluctuation in a thin sub-layer 40A, 40B can be on the order of orsmaller than a current spreading length in the n-type contact layer 16.Similarly, a thin sub-layer 40A, 40B can include compositionalinhomogeneities. The varying thickness and/or composition of a thinsub-layer 40A, 40B can result in a lateral fluctuation in the band gapof the thin sub-layer 40A, 40B in a range of one to ten thermalenergies.

Inclusion of the thin sub-layers described herein can be combined withone or more additional approaches to control stresses withinsemiconductor layers of a heterostructure. In an embodiment, the n-typecontact layer 16 can be grown using a set of growth conditionsconfigured to manipulate stresses within the heterostructure 11 in atargeted manner. For example, during growth of the n-type contact layer16, the V/III ratio can be varied to further alter the resulting tensileand compressive stresses in a thin sub-layer 40A, 40B and/or anintervening sub-layer 42A, 42B of the n-type contact layer 16.Modification of the stresses through control of the V/III ratio can beutilized to prevent relaxation in the semiconductor layer (e.g., then-type contact layer 16).

To this extent, FIG. 8 shows illustrative plots of the lattice constantsa and c as a function of the V/III ratio for an AlN layer according toan embodiment. Different lattice directions can result in differenttensile and compressive properties for the AlN layer. For example, for alow V/III ratio (e.g., less than approximately 1800), the latticeconstant a for the AlN layer is slightly larger than the latticeconstant a for an AlN layer without the presence of point defects (e.g.,approximately 3.112). The difference in the lattice constant a resultsin tensile stresses being accumulated in the layer. For a high V/IIIratio (e.g., greater than approximately 1800), the lattice constant afor the AlN layer is slightly smaller than the lattice constant a for anAlN layer without the presence of point defects, which results incompressive stresses being accumulated in the layer. The V/III ratioalso influences the lattice constant c. In this case, small values ofthe V/III ratio (e.g., below approximately 750) result in a latticeconstant c, which causes compressive stress (e.g., is belowapproximately 4.982) in the layer, while larger values of the V/IIIratio (e.g., above approximately 750) result in a lattice constant c,which causes tensile stress in the layer.

FIG. 9 shows illustrative plots of stress and strain as a function ofthe V/III ratio for an AlN layer epitaxially grown on a sapphiresubstrate according to an embodiment. As illustrated, an AlN layer grownunder a low V/III ratio (e.g., less than approximately 1800) is intensile stress, while an AlN layer grown with a high V/III ratio (e.g.,above approximately 1800) is in compressive stress. As furtherillustrated, only small changes in the strain of the AlN layer areproduced by modulating the V/III ratio. The corresponding stress in theAlN layer can be translated to the layer grown on the AlN layer (e.g.,an AlGaN intervening sub-layer). In an embodiment, growth of the n-typecontact layer 16 uses a set of growth parameters, which are adjusted todecrease local tensile stresses during growth. Furthermore, the set ofgrowth parameters can be configured to limit compressive stress duringgrowth. In an embodiment, the compressive stress is limited toapproximately 1.0 GPa or less during the growth.

Furthermore, growth of another layer in a heterostructure can utilizeone or more approaches configured to improve a quality of subsequentlygrown layers. In an embodiment, growth of a layer, such as theintermediate layer 15 (FIG. 2B), includes growth of a series ofsub-layers (e.g., films) with alternating tensile and compressivestresses, which can result in dislocation reduction in subsequentlygrown semiconductor layers, such as the n-type contact layer 16 (FIG.2B).

A layer can be selectively configured to have tensile or compressivestress by modulating a V/III ratio in each sub-layer. For example, themodulation can include varying the V/III ratio according to a setschedule to yield compressive and tensile sub-layers. Additionally, oneor more additional deposition conditions can be changed, such as agrowth temperature, a gas flow, and/or the like. Furthermore, one ormore attributes of the sub-layers, such as a relative thickness of asub-layer, a distribution of stress within each sub-layer, and/or thelike, can be adjusted during the growth of the layer. The modulation ofthe set of deposition conditions can result in regions of increasedcompressive stresses and regions of increased tensile stress. In thismanner, the resulting layer (e.g., the intermediate layer 15) can beconfigured to have a target overall residual stress (e.g., approximatelyzero or near zero).

FIGS. 10A and 10B show illustrative intermediate layers 15A, 15Baccording to embodiments. Each intermediate layer 15A, 15B is showngrown on a buffer layer 14, which can be grown on a substrate 12. In anembodiment, the substrate 12 is a foreign substrate, such as sapphire,SiC, or the like. The buffer layer 14 (e.g., a nucleation layer) canprovide a transition to accommodate a large lattice mismatch between thesubstrate 12 and the corresponding intermediate layer 15A, 15B. In anembodiment, the buffer layer 14 can comprise anAl_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N superlattice, where 0≦x, y≦1. Eachsuperlattice layer can be, for example, up to several nanometers thick.In an embodiment, the layers with differing aluminum content (e.g.,denoted by x and y) can have similar thicknesses. In an illustrativeembodiment, the buffer layer 14 has a thickness in a range from nearlyzero nanometers to approximately 2000 nanometers. In another embodiment,growth of the buffer layer 14 uses a growth temperature betweenapproximately 500 and approximately 1200 degrees Celsius and a growthrate between approximately 0.01 micrometers and approximately 10micrometers per hour.

Regardless, each intermediate layer 15A, 15B is formed of a plurality ofcompressive sub-layers 50A-50C alternating with a plurality of tensilesub-layers 52A-52C. In the intermediate layer 15A, a compressivesub-layer 50A is first grown, while in the intermediate layer 15B, atensile sub-layer 52A is first grown. While each intermediate layer 15A,15B is shown including three periods of epitaxial growth (e.g., eachperiod including a compressive and a tensile layer), it is understoodthat an intermediate layer 15A, 15B can include any number of periods.In an embodiment, the stress changes abruptly between a compressivelayer and the adjacent tensile layer. Alternatively, the stress cangradually change between adjacent layers (e.g., by growing layers havinga graded tensile or compressive stress). Furthermore, the tensile andcompressive stress can be substantially constant between periods of theintermediate layer 15A, 15B or can gradually change from period toperiod.

The growth of an intermediate layer 15A, 15B, and the growth of thecorresponding sub-layers 50A-50C, 52A-52C forming the intermediate layer15A, 15B, can use any set of deposition conditions. For example, the setof deposition conditions for a sub-layer 50A-50C, 52A-52C can include: agroup III precursor flow rate between approximately 0.1 andapproximately 200 micromoles per minute; a nitrogen precursor flow ratebetween approximately 100 and 10000 standard cubic centimeters perminute (SCCM); a pressure between approximately 1 and 760 Torr; a molarratio of group V precursors to group III precursors (V/III ratio)between approximately 10 and approximately 10000; and a growthtemperature between approximately 500 and approximately 1800 degreesCelsius. Furthermore, a sub-layer 50A-50C, 52A-52C can be grown to athickness that is greater than a critical thickness to avoidpseudomorphic growth. In an embodiment, each sub-layer 50A-50C, 52A-52Chas a thickness between approximately one nanometer and fivemicrometers.

As described herein, during the growth of an intermediate layer 15A,15B, one or more of a set of the deposition conditions for epitaxiallygrowing a sub-layer 50A-50C, 52A-52C can be changed to cause theresulting sub-layer 50A-50C, 52A-52C to exhibit either tensile orcompressive residual stress. For example, the growth of a compressivesub-layer 50A-50C and the growth of a tensile sub-layer 52A-52C can usemolar ratios of group V precursors to group III precursors that differby at least ten percent. In an embodiment, a composition of thecompressive sub-layer 50A-50C differs from a composition of the tensilesub-layer 52A-52C by no more than approximately five percent. Forexample, a fraction of aluminum in the tensile sub-layer 52A-52C candiffer from a fraction of aluminum in the compressive sub-layer 50A-50Cby no more than approximately five percent. Similarly, the compressiveand tensile sub-layers can have a lattice mismatch of at least 0.0001Angstroms. Furthermore, a growth rate for the compressive and tensilesub-layers can be changed. In an embodiment, the growth rates for thecompressive and tensile sub-layers differ by at least ten percent. Agrowth temperature for the compressive and tensile sub-layers can besubstantially the same or changed. In an embodiment, the growthtemperatures for the compressive and tensile sub-layers differ by atleast two percent. Still further, the number of and/or type ofprecursors and/or agents present during the growth of a sub-layer can beadjusted to alter elastic properties of the sub-layer. For example,precursors and agents such as: ZnO, TiN, SiN, GaAs, AlAs, GaN, InNand/or the like, can induce inhomogeneities within the sub-layer, andthus alter the elastic properties of the sub-layer.

While illustrative aspects of the invention have been shown anddescribed herein primarily in conjunction with a heterostructure for anoptoelectronic device and a method of fabricating such a heterostructureand/or device, it is understood that aspects of the invention furtherprovide various alternative embodiments.

In one embodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more of the devices designedand fabricated as described herein. To this extent, FIG. 11 shows anillustrative flow diagram for fabricating a circuit 126 according to anembodiment. Initially, a user can utilize a device design system 110 togenerate a device design 112 for a semiconductor device as describedherein. The device design 112 can comprise program code, which can beused by a device fabrication system 114 to generate a set of physicaldevices 116 according to the features defined by the device design 112.Similarly, the device design 112 can be provided to a circuit designsystem 120 (e.g., as an available component for use in circuits), whicha user can utilize to generate a circuit design 122 (e.g., by connectingone or more inputs and outputs to various devices included in acircuit). The circuit design 122 can comprise program code that includesa device designed as described herein. In any event, the circuit design122 and/or one or more physical devices 116 can be provided to a circuitfabrication system 124, which can generate a physical circuit 126according to the circuit design 122. The physical circuit 126 caninclude one or more devices 116 designed as described herein.

In another embodiment, the invention provides a device design system 110for designing and/or a device fabrication system 114 for fabricating asemiconductor device 116 as described herein. In this case, the system110, 114 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thesemiconductor device 116 as described herein. Similarly, an embodimentof the invention provides a circuit design system 120 for designingand/or a circuit fabrication system 124 for fabricating a circuit 126that includes at least one device 116 designed and/or fabricated asdescribed herein. In this case, the system 120, 124 can comprise ageneral purpose computing device, which is programmed to implement amethod of designing and/or fabricating the circuit 126 including atleast one semiconductor device 116 as described herein.

In still another embodiment, the invention provides a computer programfixed in at least one computer-readable medium, which when executed,enables a computer system to implement a method of designing and/orfabricating a semiconductor device as described herein. For example, thecomputer program can enable the device design system 110 to generate thedevice design 112 as described herein. To this extent, thecomputer-readable medium includes program code, which implements some orall of a process described herein when executed by the computer system.It is understood that the term “computer-readable medium” comprises oneor more of any type of tangible medium of expression, now known or laterdeveloped, from which a stored copy of the program code can beperceived, reproduced, or otherwise communicated by a computing device.

In another embodiment, the invention provides a method of providing acopy of program code, which implements some or all of a processdescribed herein when executed by a computer system. In this case, acomputer system can process a copy of the program code to generate andtransmit, for reception at a second, distinct location, a set of datasignals that has one or more of its characteristics set and/or changedin such a manner as to encode a copy of the program code in the set ofdata signals. Similarly, an embodiment of the invention provides amethod of acquiring a copy of program code that implements some or allof a process described herein, which includes a computer systemreceiving the set of data signals described herein, and translating theset of data signals into a copy of the computer program fixed in atleast one computer-readable medium. In either case, the set of datasignals can be transmitted/received using any type of communicationslink.

In still another embodiment, the invention provides a method ofgenerating a device design system 110 for designing and/or a devicefabrication system 114 for fabricating a semiconductor device asdescribed herein. In this case, a computer system can be obtained (e.g.,created, maintained, made available, etc.) and one or more componentsfor performing a process described herein can be obtained (e.g.,created, purchased, used, modified, etc.) and deployed to the computersystem. To this extent, the deployment can comprise one or more of: (1)installing program code on a computing device; (2) adding one or morecomputing and/or I/O devices to the computer system; (3) incorporatingand/or modifying the computer system to enable it to perform a processdescribed herein; and/or the like.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A heterostructure comprising: a n-type contactlayer including: a plurality of intervening sub-layers of a first groupIII nitride material and having a first thickness; and a plurality ofthin sub-layers of a second group III nitride material and having asecond thickness, wherein the plurality of intervening sub-layersalternate with the plurality of thin sub-layers, wherein the first groupIII nitride material includes a molar fraction of gallium of at least0.05, wherein a molar fraction of gallium in the first group III nitridematerial exceeds the molar fraction of gallium in the second group IIInitride material by at least 0.05, and wherein the second thickness isat most five percent of the first thickness.
 2. The heterostructure ofclaim 1, further comprising a substrate and a buffer layer locateddirectly on the substrate, wherein the n-type contact layer is locatedon the buffer layer.
 3. The heterostructure of claim 1, furthercomprising an active region located directly on the n-type contactlayer.
 4. The heterostructure of claim 1, wherein the second thicknessis between approximately 0.2 nanometers and approximately 2 nanometers.5. The heterostructure of claim 1, wherein the first group III nitridematerial includes aluminum having an aluminum molar fraction of at least0.1, and wherein the first thickness is within fifty percent of thealuminum molar fraction multiplied by 100 nanometers.
 6. Theheterostructure of claim 1, wherein the first thickness is betweenapproximately 20 nanometers and approximately 500 nanometers.
 7. Theheterostructure of claim 1, wherein the second group III nitridematerial is aluminum nitride.
 8. The heterostructure of claim 1, whereinthe first group III nitride material is Al_(x)Ga_(1−x)N with 0.4<x<0.7.9. The heterostructure of claim 1, wherein the n-type contact layerincludes at least twenty thin sub-layers.
 10. A device comprising: asubstrate; and an n-type layer formed of group III nitride materialslocated on the substrate, wherein the n-type layer includes: a pluralityof intervening sub-layers of a first group III nitride material andhaving a first thickness; and a plurality of thin sub-layers of a secondgroup III nitride material and having a second thickness, wherein theplurality of intervening sub-layers alternate with the plurality of thinsub-layers, wherein the first group III nitride material includes amolar fraction of gallium of at least 0.05, wherein a molar fraction ofgallium in the first group III nitride material differs from the molarfraction of gallium in the second group III nitride material by at least0.05, and wherein the second thickness is at most five percent of thefirst thickness, wherein the n-type layer is one of: an n-type contactlayer, an n-type cladding layer, or a carrier electronic layer.
 11. Thedevice of claim 10, wherein the device is configured to operate as alight emitting diode, the device further including an active regionadjacent to the n-type layer, wherein the n-type layer is one of: then-type contact layer or the n-type cladding layer.
 12. The device ofclaim 11, wherein the plurality of intervening sub-layers have a molarfraction of aluminum within approximately twenty percent of a molarfraction of aluminum of a first barrier in the active region adjacent tothe n-type layer.
 13. The device of claim 10, wherein the device isconfigured to operate as a high electron mobility transistor, whereinthe n-type layer is a carrier electronic layer.
 14. The device of claim10, wherein the second thickness is less than a tunneling length ofcarriers in the n-type layer.
 15. The device of claim 10, wherein thefirst group III nitride material is AlGaN with a molar fraction ofaluminum between 0.4 and 0.7, wherein the second group III nitridematerial is AlN, wherein the first thickness is in a range of 20nanometers to 500 nanometers, and wherein the second thickness is in arange of 0.2 nanometers to 2 nanometers.
 16. The device of claim 15,wherein at least one of: the molar fraction of aluminum, the firstthickness, or the second thickness, increases or decreases along thelateral height of the n-type layer.
 17. A method of fabricating adevice, the method comprising: forming a n-type layer on a substrate,wherein the n-type layer includes: a plurality of intervening sub-layersof a first group III nitride material and having a first thickness; anda plurality of thin sub-layers of a second group III nitride materialand having a second thickness, wherein the plurality of interveningsub-layers alternate with the plurality of thin sub-layers, wherein thefirst group III nitride material includes a molar fraction of gallium ofat least 0.05, wherein a molar fraction of gallium in the first groupIII nitride material exceeds the molar fraction of gallium in the secondgroup III nitride material by at least 0.05, and wherein the secondthickness is at most five percent of the first thickness.
 18. The methodof claim 17, wherein the first group III nitride material is AlGaN witha molar fraction of aluminum between 0.4 and 0.7, wherein the secondgroup III nitride material is AlN, wherein the first thickness is in arange of 20 nanometers to 500 nanometers, and wherein the secondthickness is in a range of 0.2 nanometers to 2 nanometers.
 19. Themethod of claim 17, further comprising selecting the second thickness tolimit a sheet resistivity of the n-type layer to no more than fiftypercent more than a sheet resistivity of a comparable n-type layerformed of the first group III nitride material, having a same thicknessas a thickness of the n-type layer, and including no thin sub-layers.20. The method of claim 17, further comprising selecting the firstthickness and a total number of the plurality of thin sub-layers toreduce an amount of wafer bowing by at least ten percent over acomparable n-type layer formed of the first group III nitride material,having a same thickness as a thickness of the n-type layer, andincluding no thin sub-layers.